The reverse air cycle type of heat pump utilizes unidirection refrigerant flow wherein the condenser and evaporator retain their functions, but the air directed across them is redirected for different operations. While the heat pump is operating in the cooling mode, outdoor air is passed in heat exchange relationship with the condenser for liquifying the refrigerant and outside again; and indoor air is passed in heat exchange relationship with the evaporator for cooling the air circulated again. Conversely, in the heating mode, outdoor air passes in heat exchange relationship with the evaporator for vaporizing the refrigerant, then outside again; and indoor air is passed in heat exchange relationship with the evaporator for vaporizing the refrigerant, then outside again; and indoor air is passed in heat exchange relationship with the condenser for heating the air and circulated again.
One prior art U.S. Pat. No. 2,878,657-Atchison, assigned to General Electric Company, the assignee of the present invention, discloses a heat pump wherein the air conditioning unit includes a plurality of air controlling valves each of which is associated with an opposed inlet and outlet opening of the units that permit selective control of the air flowing into the discharging from the unit in order to direct air either from the outside or from within the enclosure over either of the heat exchangers disposed within separate compartments of the unit.
Under certain operating conditions in the heating cycle, evaporator may operate at such low outdoor ambient temperatures as to cause the accumulation of a coating or layer of frost on its surface. Since frost when it accumulates operates as a barrier to heat transfer between the evaporator and the air being circulated thereover, the efficiency of the unit is markedly reduced. Further, unless means are provided for interrupting the accumulation of frost, the evaporator can become completely filled with a layer of frost that may effectively block air passage therethrough. This blockage of air results in the loss of heat exchange and if allowed to continue can cause refrigeration system components to fail and can also result in compressor burn-out unless compressor operation is terminated.
The shutting down of compressor operation each time frost accumulates severely curtails the operation of the unit in the heating cycle and accordingly the efficiency of the unit as a heating means at temperatures below the evaporator frosting level.
In U.S. Pat. No. 3,555,842-Bodcher, a defrost line connects the upper inlet of the condenser to the upper inlet of the evaporator and includes a defrost valve which is closed during operation of the compressor but opens when compressor operation terminates. A return line connects the evaporator collector with the lower part of the condenser and includes a valve which operates in the same manner as the defrost valve.
In U.S. Pat. No. 4,158,950-McCarty, assigned to the General Electric Company, assignee of the present invention, there is disclosed a defrost arrangement for refrigeration system of the reverse cycle. A secondary defrost circuit is provided which permits refrigerant flow to by-pass the compressor when compressor operation terminates.